ZyXEL NAS products running firmware version 5.21 and earlier are vulnerable to pre-authentication command injection in weblogin.cgi

Description

Multiple ZyXEL network-attached storage (NAS) devices running firmware version 5.21 contain a pre-authentication command injection vulnerability, which may allow a remote, unauthenticated attacker to execute arbitrary code on a vulnerable device. ZyXEL NAS devices achieve authentication by using the weblogin.cgi CGI executable. This program fails to properly sanitize the username parameter that is passed to it. If the username parameter contains certain characters, it can allow command injection with the privileges of the web server that runs on the ZyXEL device. Although the web server does not run as the root user, ZyXEL devices include a setuid utility that can be leveraged to run any command with root privileges. As such, it should be assumed that exploitation of this vulnerability can lead to remote code execution with root privileges. By sending a specially-crafted HTTP POST or GET request to a vulnerable ZyXEL device, a remote, unauthenticated attacker may be able to execute arbitrary code on the device. This may happen by directly connecting to a device if it is directly exposed to an attacker. However, there are ways to trigger such crafted requests even if an attacker does not have direct connectivity to a vulnerable devices. For example, simply visiting a website can result in the compromise of any ZyXEL device that is reachable from the client system. Affected products include: NAS326 before firmware V5.21(AAZF.7)C0 NAS520 before firmware V5.21(AASZ.3)C0 NAS540 before firmware V5.21(AATB.4)C0 NAS542 before firmware V5.21(ABAG.4)C0 ZyXEL has made firmware updates available for NAS326, NAS520, NAS540, and NAS542 devices. Affected models that are end-of-support: NSA210, NSA220, NSA220+, NSA221, NSA310, NSA310S, NSA320, NSA320S, NSA325 and NSA325v2

Scores

EPSS (exploitation in 30 days)

100.0% · top 0.1% of all CVEs (6 Nov 2026)

CISA SSVC

Exploitation: active · Automatable: no · Technical impact: total

The list uses the assigner's score when there is one, then CISA's, then NVD's: Critical 9.8.

Weakness

• CWE-78 OS Command Injection (opens in a new tab)All about CWE-78 and its CVEs

Affected products

• ZyXEL · NAS326

  • affected V5.21(AAZF.7)C0 – ≤ V5.21(AAZF.7)C0

• ZyXEL · NAS520

  • affected V5.21(AASZ.3)C0 – ≤ V5.21(AASZ.3)C0

• ZyXEL · NAS540

  • affected V5.21(AATB.4)C0 – ≤ V5.21(AATB.4)C0

• ZyXEL · NAS542

  • affected V5.21(ABAG.4)C0 – ≤ V5.21(ABAG.4)C0

• ZyXEL · NSA210

  • affected all

• ZyXEL · NSA220

  • affected all

• ZyXEL · NSA220+

  • affected all

• ZyXEL · NSA221

  • affected all

• ZyXEL · NSA310

  • affected V4.75(AALH.2)C0 – ≤ V4.75(AALH.2)C0

• ZyXEL · NSA320

  • affected all

• ZyXEL · NSA320S

  • affected V4.75(AANV.2)C0 – ≤ V4.75(AANV.2)C0

• ZyXEL · NSA325

  • affected V4.81(AAAJ.1)C0 – ≤ V4.81(AAAJ.1)C0

• ZyXEL · NSA325v2

  • affected V4.81(AALS.1)C0 – ≤ V4.81(AALS.1)C0

What to do

Solution: ZyXEL has made firmware updates available for NAS326, NAS520, NAS540, NAS542, ATP100, ATP200, ATP500, ATP800, USG20-VPN, USG20W-VPN, USG40, USG40W, USG60, USG60W, USG110, USG210, USG310, USG1100, USG1900, USG2200, VPN50, VPN100, VPN300, VPN1000, ZyWALL110, ZyWALL310, and ZyWALL1100 devices.

Workaround: Block access to the ZyXEL device web interface: This issue can be mitigated by blocking (for example with a firewall) access to the web interface (80/tcp and 443/tcp) of any vulnerable ZyXEL device. Any machine that can access the ZyXEL web interface should not also be able to access the internet. Restrict access to vulnerable ZyXEL devices: Direct exploitation of this vulnerability can be mitigated by restricting access to vulnerable devices. In particular, do not expose such devices directly to the internet. Note however, that it is still possible for attackers to exploit devices that are not directly connected to the internet. For example, by way of viewing a web page.

Exploitation: https://kb.cert.org/artifacts/cve-2020-9054.html

References (6)

• https://cwe.mitre.org/data/definitions/78.html (opens in a new tab)x_refsource_MISCx_transferred
• https://www.zyxel.com/support/remote-code-execution-vulnerability-of-NAS-products.shtml (opens in a new tab)x_refsource_CONFIRMx_transferred
• VU#498544 (opens in a new tab)third-party-advisoryx_refsource_CERT-VNx_transferred
• https://kb.cert.org/artifacts/cve-2020-9054.html (opens in a new tab)x_refsource_MISCx_transferred
• https://krebsonsecurity.com/2020/02/zyxel-fixes-0day-in-network-storage-devices/ (opens in a new tab)x_refsource_MISCx_transferred
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2020-9054 (opens in a new tab)government-resource

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• CISA KEV catalog (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisories (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2020-9054](https://trackcve.app/badge/CVE-2020-9054.svg)](https://trackcve.app/cve/CVE-2020-9054)

Record last updated 7 Oct 2026, 17:32 UTC.